The present invention relates to yieldably extensible, self-retracting shielded cables, and particularly to such cables which are capable of carrying high frequency signals.
Yieldably extensible, self-retracting shielded cables have been known in the past as evidenced, for example, by the constructions shown in Maddox U.S. Pat. No. 3,240,867 and Timmons U.S. Pat. No. 3,274,329. Such previous cables employ exterior heat-settable dielectric jackets, i.e., either thermoplastic jackets which are heated to their plasticizing temperatures and cooled or thermosetting jackets which are heated to their curing temperatures, while held in a coiled configuration in order to form the permanently-coiled shape which provides the desired yieldable retractability. The primary electrical insulation between the central conductors and shields of such previous self-retracting cables is normally rubber or plastic having a high enough melting point (or other degradation point) that the location of the central conductor relative to the shield, and the electrical properties of the insulation, are not changed by the heating of the exterior jacket to its plasticizing or curing temperature as the case may be. Such insulation also has sufficient mechanical strength that the electrical properties of such insulation are not significantly affected by the kinking and distortion of a surrounding braided wire shield caused by the coiled configuration. Unfortunately, such insulation materials which are thermally and mechanically resistant to the jacket heating procedure and to the kinking of the shield have a relatively high dielectric constant unsuitable for transmission of high frequency signals.
Alternative electrical insulation materials having substantially lower dielectric constants suitable for the transmission of high-frequency signals have been available for some time. These are primarily expanded, stretched or foamed materials, such as polymeric fluorocarbon, which are relatively porous in order to produce a low dielectric constant but which, as a result of their porosity, do not have as high mechanical strengths as those insulating materials of higher density and higher dielectric constant. Although these low-dielectric-constant insulating materials have been used successfully in straight shielded cables as exemplified by Sass U.S. Pat. No. 4,552,989, they have not successfully been employed in permanently coiled, yieldably extensible and retractable cables for two significant reasons: first, some of them have relatively low melting points (or other degradation points) so that subjecting them to the plasticizing or curing temperature of the exterior jacket would degrade their electrical characteristics and/or change the location of the conductor or conductors relative to the shield; second, their relatively fragile mechanical properties cause their electrical characteristics likewise to be adversely affected if subjected to kinking or distortion of a surrounding braided wire shield having a coiled configuration.
Unbraided helical shields wrapped in a single direction as shown, for example, in Timmons U.S. Pat. No. 3,274,329, while being less likely to kink or distort and thus less likely to affect the more fragile insulating materials, cause excessive inductance in the shield and thus distort high-frequency transmissions. Although wire shields composed of inner and outer layers of unbraided wire helically wound in opposite directions have also been employed in the past, as exemplified by Martin U.S. Pat. No. 3,334,177, Felkel U.S. Pat. No. 4,131,757 and Ziemek U.S. Pat. No. 4,738,734, they have not been employed advantageously in permanently coiled, extensible and retractable cables.
Accordingly, what is needed is a cable construction which renders the use of low-dielectric-constant insulating materials, for high-frequency signal transmissions, compatible with a permanently-coiled extensible and retractable cable configuration having a shield likewise suitable for high-frequency transmissions.